This invention relates to absorbing arrangements and methods for temporarily storing oxides of nitrogen and sulfur and removing such oxides from the exhaust gas from internal combustion engines.
As used herein, the term xe2x80x9cabsorbxe2x80x9d includes the chemical process for storing gases such as, for example, by conversion of barium oxide to barium nitrate for storage of nitrogen oxide.
U.S. Pat. No. 4,755,499 discloses an arrangement for the reversible storage of oxides of nitrogen and sulfur, for example from motor vehicle exhaust gases, in which the absorber is regenerated by heating in a reducing atmosphere. In this arrangement, a reduction of the nitrogen oxides takes place at the same time.
A storage catalyst of that type for use in motor vehicles is described in more detail in U.S. Pat. No. 5,402,641, in which high temperatures above 500xc2x0 C. are necessary to regenerate the absorber. Consequently, use of the storage catalyst is possible only for motor vehicles having a high exhaust-gas temperature, in particular for motor vehicles with an Otto engine.
In this case, however, the possibility of use is limited since, under certain operating conditions of internal combustion engines, such as occur for example in city traffic, the acceleration phases cause a large emission of nitrogen oxide, but no long-lasting high temperature condition such as is required to regenerate the absorber, especially with respect to oxides of sulfur, is attained.
Accordingly, it is an object of the present invention to provide a method and arrangement for releasably absorbing oxides of sulfur and nitrogen which overcomes disadvantages of the prior art.
Another object of the invention is to provide an absorber for nitrogen oxides and a corresponding method, suitable especially for use with fuel consumption-optimized engines such as Diesel and/or direct injection Otto engines, in which regeneration of the absorber is possible even at low exhaust gas temperatures.
These and other objects of the invention are attained by providing an arrangement for absorbing oxide gases which includes an absorption layer for absorbing oxides of nitrogen and/or sulfur and a support for the absorption layer which has a wall thickness xe2x89xa60.16 mm. The support preferably has a wall thickness xe2x89xa60.1 mm and desirably xe2x89xa60.05 mm. The support may be a metal sheet or foil and may be heatable by application of an electric current. Preferably the support is formed so as to provide flow passages with a structure arranged to make the flow turbulent. With this arrangement, regeneration of the absorber is possible even at very low exhaust gas temperatures such as occur for example in the case of direct injection Diesel engines.
According to the invention, the usual gas absorbing materials may be employed, for example as described in U.S. Pat. No. 4,755,499, and also in U.S. Pat. Nos. 5,402,641 and 5,362,463. A common feature of all these storage materials is that they have an elevated absorption temperature, while a still higher regeneration temperature is required especially for removing the oxides of sulfur. For most storage media of this kind, temperatures in the range from 150xc2x0 to 700xc2x0 C., in particular temperatures above 300xc2x0 C., are required. Such temperatures commonly occur in motor vehicles with Otto engines, but are comparatively rare with Diesel engines and especially in internal combustion engines having direct fuel injection.
The preferred NOx storage materials are distinguished in that, under conditions of net oxidation, i.e., a stoichiometric excess of oxidizing agents, such as occurs in the exhaust gas during the operation, they will store nitrogen oxides and, upon a reduction of the excess of oxygen, may reduce them. For this purpose, the NOx storage catalysts usually include a precious metal, in particular the usual precious metal coatings for three-way catalysts. The NOx-laden storage material is advantageously regenerated in a regenerating phase at xcexxe2x89xa61.
Ordinarily, various reactions take place successively or simultaneously on the NOx storage catalyst, the most important reactions being: oxidation of the NO in the exhaust gas to NO2, storage of the NO2 as nitrate, decomposition of the nitrate, and reduction of the re-formed NO2 to nitrogen and oxygen.
As described above, the course of the reactions depends, among other things, not only on the temperature of the catalyst but also on the concentration of the reagents at the active region of the catalyst and the flow velocity of the gas.
According to the invention, it has now been found that, with various factors capable of being combined with each other, it is possible also, at little cost, to optimize the known exhaust-gas absorbers so that they may be employed for internal combustion engines with direct injection and for Diesel engines. For this purpose, the wall thickness of the supporting member on which the absorption layer is applied preferably should be xe2x89xa6160 microns, and desirably xe2x89xa6140 microns and if a metal support is used, a wall thickness xe2x89xa650 microns, preferably xe2x89xa640 microns, and desirably xe2x89xa630 microns, and the absorber should preferably be heated to a temperature above the temperature of the exhaust gas.
According to the invention, it has been found that, with the use of thin-walled ceramic supports for the absorption layer, i.e. supporting members having a wall thickness xe2x89xa60.14 mm, not only is a more rapid temperature rise of the absorption layer possible, but also a thicker absorption layer may be used. This accomplishes two objectives: in the first place, even short periods of high-temperature operation can be utilized for regeneration since the storage layer temperature will be increased to the required temperature more quickly, and in the second place, by providing a thicker absorption layer, a higher NOx or SOx storage capacity can be achieved so that during operation of the internal combustion engine a longer period of time can elapse before the storage layer must be regenerated. Consequently, despite the less frequent occurrence of temperature peaks in the exhaust gas of consumption-optimized internal combustion engines, no failure of the storage layer resulting from exceeding its storage saturation limit will occur.
According to the invention, absorbers having a support member made of metal foil are especially suitable, and the metal foil may advantageously be connectable to an electric power source for resistance heating so that, even at low exhaust gas temperatures, the absorber can be brought to the necessary regenerating temperature by passing an electric current through the metal support. Furthermore, by using a metal support member, the gas passages which are coated with the absorption layer may be variously shaped, so that, for example, a controlled turbulent flow vortex of the exhaust gas in the passages can be established.
With especial advantage, according to the invention, supports with a variety of passage segments may be used for the absorber where, for example, an intermediate segment of the passages is modified to produce a turbulent flow. This can be done, for example, by varying the passage cross-section, or by a twisting or distortion of the passages. In this way, the support may be adapted in a controlled way for especially favorable reaction conditions along the flow passages. Another special feature of the support, beside a possible variation in number of passages in the flow direction and the provision of changes of cross section along the flow direction, is the segmentation of the support where, for example, one segment with an absorption layer is disposed near the engine outlet and another segment with an absorption layer is located somewhat farther away. Thus, even with the most variable operating conditions, good NOx purification results can be obtained with fuel consumption-optimized engines.
According to the invention, it has been found that the NOx storage arrangement will have especially good absorption and desorption properties if the flow passages for the exhaust gas are distorted in an intermediate region to achieve a turbulent flow between an inlet region and an outlet region which do not have a distorted structure to produce turbulent flow. As a simple arrangement for generating such a turbulent flow, for example, a transition from a large to a small diameter in the passages is effective, but twisting of the entire support in an intermediate region will also serve to generate turbulence. The especially favorable properties resulting from such turbulent flow are presumably achieved by a division of the individual reaction steps required to reduce nitrogen oxides among the successive regions of the support, with a modified intermediate region affording better conditions for reaction than unmodified intermediate regions.
To produce especially good oxide gas conversions, the absorption layer has an enlarged surface area, that is, a total surface area that is substantially larger than the area of the surface of the support member on which it is coated. For this purpose, the absorption layer provides a surface area of at least 20 m2/g, and preferably at least 40 m2/g. Also, the absorption layer preferably has a pore volume of at least 0.2 cm3/g, and desirably at least 0.4 cm3/g, a bimodal pore size distribution with both micropores and macropores also being acceptable. This may be achieved for example by the choice of the size of the particles forming the absorber surface, in which mixtures or specified distributions of different particle sizes are also suitable.
An especially suitable absorption material is gamma-aluminum oxide containing one or more elements in the group consisting of alkali metals, alkaline-earth metals, rare earths and/or lanthanum. The presence of the elements copper and manganese is also suitable. The added elements are usually present as oxides, or else as carbonates or nitrates, the storage effects being achieved by formation of corresponding nitrates and sulfates, which are then converted back to oxides or carbonates under the appropriate reaction conditions. In this way, it is possible to absorb NOx and/or SOx from an exhaust gas containing at least 1% oxygen.
As described above, the absorbed substances are desorbed from the storage catalyst layer by elevated temperatures and in a reducing atmosphere. For this purpose, it is desirable to determine the oxygen concentration in the exhaust gas so that the oxygen concentration, or a quantity having a known relationship to the oxygen concentration, can be utilized to control the process of absorption or desorption.
Since the temperature of the absorption layer, determined directly or indirectly, is also important the same consideration is also applicable to the temperature of the exhaust gas. Thus, the absorption layer temperature may for example be determined by measuring the temperature of the exhaust gas or of the support member. A determination of temperature over the operating diagram of the internal combustion engine is also possible.
With the present invention, absorption layers having a thickness of at least 50 microns, preferably at least 70 microns, and desirably at least 90 microns, can be provided. These values are average layer thickness of a cross section and should extend over preferably at least 50% and desirably at least 80% of the total absorber. The foregoing absorbtion layer thickness values apply to layers on ceramic substrates. Half of those values apply to absorbtion layers provided on metal substrates. Such high layer thickness values permit a greater storage capacity compared to conventional absorbers, and consequently permit longer intervals between regeneration as described above.
According to the method of the invention, regeneration of the absorbtion layer is preferably carried out when the operating conditions of the internal combustion engine produce a correspondingly high temperature of the exhaust gas and hence of the absorption layer. Especially advantageous, however, is a method in which supplementary heating of the absorption layer is provided, preferably electrically. Other possible heating procedures include ignition control measures in Otto engines, variation of lambda, lowering of lambda below 1, and addition of secondary air to generate exothermia on an oxidation catalyst, and/or an exhaust ignition arrangement, as well as heating the catalyst with a burner. A segmented absorber in which the segments are heated according to the required reaction is especially advantageous. Thus, for example, only one absorber segment located in a downstream exhaust flow direction may be heated, especially in case of a distinct spatial separation of the absorber segments. Electric heating is especially advantageous in this case, but an injection of fuel into the exhaust gas and/or a burner may also be used. By arranging individual segments for individual reactions at different distances from the engine exhaust manifold, thermal aging of the absorber may be reduced in addition to providing the advantage of especially favorable reaction temperatures in individual absorber segments.
Further, by heating the absorber such as electrically or by means of a burner, a soot deposition the absorber which would otherwise definitely reduce the storage capacity can be burned off. For this purpose, the adjustment of the lambda in the exhaust gas flow to a value not over 1 is preferably performed after burning off the carbon deposits.
Since the release and conversion of the NOx from the storage layer and the release of the oxides of sulfur from the storage layer require different temperatures, higher in the case of the sulfur oxides, it is also possible to proceed so that a desorption of the oxides of sulfur, which are present in particular as sulfate is performed at longer time intervals or only as needed, so that the storage layer is only occasionally heated to the high temperatures needed for desorption of the sulfur oxides. This counteracts premature aging of the storage layer, so that an especially good long-term stability of the absorber is achieved. This procedure may be used with the absorber arrangement and method described above.